The spine is composed of several individual bones, known as vertebrae. The vertebrae vary in size, shape, and function in different regions of the spine. The cervical vertebrae make up the bones of the neck and provide for much of the movement of the head. The thoracic vertebrae act as anchors for the ribs and are relatively immobile. The lumbar vertebrae, located at the base of the spine, are the largest vertebrae and allow movement and articulation of the trunk.
In between adjacent vertebrae is an intervertebral disc, which consists of a fibrous outer portion and a gelatinous inner portion. The discs allow the vertebrae to move and articulate relative to one another. They also act as a shock absorber when there is a blow to the spine, such as from a fall or a strike on the head. In particular, an intervertebral disc is capable of at least six different motions or degrees of freedom: flexion, which is bending forward from the waist; extension, which is bending backward from the waist; lateral bending, or leaning sideways; axial rotation, which is turning or twisting to one side or the other; axial deflection, which may also be known as axial compression, vertical extension, or compression along the spine; and anterior/posterior (A/P) translation, which is when one vertebrae slides forward or backward relative to an adjacent vertebra without changing its angle relative to the neighboring vertebra.
The intervertebral disc can be injured or damaged as a result of aging, trauma, or disease. The disc can become desiccated or otherwise loosen or weaken in structure, a condition known as degenerative disc disorder (DDD). A herniated disc is one that has developed a tear in the outer portion, allowing the inner portion (e.g., the internal gelatinous portion) to push out. In any case, a damaged disc no longer permits movement as it once did, which may cause pain and/or discomfort to a patient. For example, as the vertebrae move out of their normal, healthy position, the patient may develop chronic, and in some case debilitating, pain as nerves are compressed.
Historically, the injured disc, in a condition such as DDD or herniated disc, can be treated with spinal fusion. Spinal fusion can also be indicated as a treatment for tumors, fractures, and conditions such as scoliosis and kyphosis. In the fusion procedure, a discectomy is performed to remove the damaged disc and the adjacent vertebrae are physically joined together with rods, wire, or other instrumentation. A bone graft may be placed between the vertebrae, and over several months, the vertebrae grow together. A typical fusion patient does not notice any loss in mobility because her range of motion was even more restricted by the original condition or injury.
Nevertheless, the lack of motion between the fused vertebrae places increased stress on the surrounding vertebrae and intervertebral discs. This increased stress may lead to premature failure or injury to these components, requiring further treatment. In addition, a fusion procedure may be a major operation, requiring open back surgery and a long recovery period. For these reasons, it is typically a treatment of last resort, reserved for severe cases or when other treatment options have failed.
Alternatives to the open spine fusion procedure, including minimally invasive procedures and artificial disc replacements, are in various stages of development and practice, but these alternatives have yet to see widespread adoption. Minimally invasive procedures involve the use of small incisions, remote control manipulation of instruments, and observation through an endoscope or similar access device. These procedures may result in less trauma to the patient and improved recovery times. Minimally invasive surgery can also be used to replace an injured intervertebral disc. Instead of fusing the vertebrae above and below a damaged or diseased disc, the disc can be replaced with an artificial or prosthetic disc, for example. Current prosthetic discs may provide a greater range of motion than an equivalent fusion procedure while offering equal or better treatment of the condition.
Current artificial discs, however, may have a plurality of interlocked components that may be subject to inadvertent separation after surgical procedures. Such disassembly may require additional procedures to correct. Accordingly, there is a need for an artificial disc that enables all six degrees of movement, restricts movement along one or more degrees, is compatible with a non-anterior surgical procedure, and has increased durability.